Method of growing mono crystalline tubular bodies from the melt

ABSTRACT

The invention is a method of growing a monocrystalline extension such as an end wall or flange onto the end of a monocrystalline tube. Typically the tube is made of alumina.

United States Patent 11 1 1111 3,915,662

Labelle et al. Oct. 28, 1975 METHOD OF GROWING MONO 264/164 CRYSTALLINETUBULAR BODIES FROM THE MELT [56] References Cited :75] Inventors:Harold E. Labelle, Hanover; Charles UNITED STATES PATENTS J- Cronan, non, h f Ma 3,765,843 10/1973 Labelle et a1 23/301 SP 3,801,309 4/1974Mlavsky 1 v 23/30] SP Assgnee' Labmatones 3,826,625 7 1974 Bailey 23/301SP Mass.

22] Filed: Aug. 15, 1973 Primary Examiner-Norman Yudkoff AssistantExaminer-D. Sanders Appl' 388597 Attorney, Agent, or Firm-Schiller &Pandiscio Related US. Application Data 60] Division of Ser. No. 165,087,July 23, 1971, [57] ABSTRACT abandoned which is a of The invention is amethod of growing a monocrystal- 144920 May 1971 abardoned' lineextension such as an end wall or flange onto the end of amonocrystalline tube. Typically the tube is 52] US. Cl 23/301 SP; 23/273SP; 264/164 51 1111. C1. B01J 17/20 made 31mm 58] Field of Search 23/301SP, 273 SP; 10 Claims, 11 Drawing Figures US. Patent Oct.28, 1975 v.Sheet10f3 3,915,662

PULLING MECHANISM US. Patent Oct. 28, 1975 Sheet20f3 3,915,662

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METHOD OF GROWING MONO CRYSTALLINE TUBULAR BODIES FROM TIIE MELT Thisapplication is a division of our copending application Ser. No. 165,087filed July 23, 1971 now abandoned which in turn is acontinuation-in-part of a copending application Ser. No. 144,920, filedMay 19, 1971 now abandoned.

This invention relates to production of substantially monocrystallinetubular bodies having end walls or flanges.

The present invention pertains to an improvement in growing crystallinebodies from the melt according to what is called the edge-defined,film-fed growth technique (also known as the EFG process). Details ofthis process are described in the copending U.S. Pat. application ofHarold E. LaBelle, Jr., Ser. No. 700,126 filed .Ian. 24, 1968 for Methodof Growing Crystalline Materials.

In the EFG process the shape of the crystalline body is determined bythe external or edge configuration of the end surface of a formingmember which for want of a better name is called a die. An advantage ofthe process is that bodies of selected shapes such as round tubes orflat ribbons can be produced commencing with the simplest of seedcrystal geometries, namely, a round small diameter seed crystal. Theprocess involves growth on a seed from a liquid film of feed materialsandwiched between the growing body and the end surface of the die, withthe liquid in the film being continuously replenished from a suitablemelt reservoir via one or more capillaries in the die member. Byappropriately controlling the pulling speed of the growing body and thetemperature of the liquid film, the film can be made to spread (underthe influence of the surface tension at its periphery) across the fullexpanse of the end surface of the die until it reaches the perimeter orperimeters thereof formed by intersection of that surface with the sidesurface or surfaces of the die. The angle of intersection of theaforesaid surfaces of the die is such relative to the contact angle ofthe liquid film that the liquids surface tension will prevent it fromoverrunning the edge or edges of the dies end surface. Preferably theangle of intersection is a right angle which is simplest to achieve andthus most practical to have. The growing body grows to the shape of thefilm which conforms to the edge configuration of the dies end surface.Since the liquid film has no way of discriminating between an outsideedge and an inside edge of the dies end surface, a continuous hole maybe grown in the crystalline body by providing in that surface a blindhole of the same shape as the hole desired in the growing body,provided, however, that any such hole in the dies end surface is madelarge enough so that surface tension will not cause the film around thehole to fill in over the hole. From the foregoing brief description itis believed clear that the term edge-defined, film-fed growth denotesthe essential feature of the EFG process-the shape of the growingcrystalline body is defined by the edge configuration of the die andgrowth takes place from a film of liquid which is constantalyreplenished. v

The primary object of the present invention is to provide a method,using the aforesaid EFG process, of growing substantiallymonocrystalline extensions of selected shape on substantiallymonocrystalline tubes.

Another object is to provide essentially monocrystalline tubes havingsubstantially monocrystalline end walls or flanges.

In this connection it is to be noted that the EFG process may be used togrow monocrystalline tubes of selected ceramic materials such as aluminaand that tubes made of polycrystalline or substantially monocrystallinealumina have utility as envelopes for high intensity vapor lamps. In themanufacture of such lamps the practice is to mount the electrodes in endcaps that are attached to the ends of the envelopes by brazing or othersuitable technique. It is recognized that mounting of the electrodes maybe facilitated by forming the tubes with end walls each having anopening for direct mounting of an electrode without need for an end cap.Alternatively the tubes may be formed with inner or outer end flanges tofacilitate attachment of end caps. It also is dedirable for otherapplications to form ceramic tubes each having an imperforate end wallat one end. However, heretofore it has not been possible to formmonocrystalline tubes having integral end walls or flanges of likematerial and crystallinity, and particularly end flanges of closelycontrolled diameter and thickness. Accordingly a more specific object ofthis invention is to provide a method of producing monocrystalline tubesof ceramic materials such as alphaalumina that terminate in integral endwalls or flanges.

Described briefly, the method of this invention comprises taking apreviously grown monocrystalline tube of a selected material such asalumina and growing a monocrystalline extension such as an end wall orflange onto the end thereof by the EFG process using a die member havinga film supporting end surface that conforms in shape to the desiredshape of the extension to be grown.

Other features and many of the attendant advantages of this inventionare set forth or rendered obvious in the following detailed descriptionwhich is to be consid ered together with the accompanying drawingswherein:

FIG. 1 is a fragmentary elevational view, partly in section of apparatuscomprising a furnace, crucible and die assembly used in practicing theinvention;

FIG. 2 is an enlarged view of a portion of the apparatus of FIG. 1showing the initial step in growing a closed end wall on a tube;

FIGS. 3-5 are views similar to FIG. 2 illustrating how crystal growthoccurs in forming an end wall on a tube;

FIGS. 69 show how an internal flange is grown on the end of a tube; and

FIGS. 10 and 11 illustrate how an end wall may be grown using amodification of the die assembly shown in FIG. 2.

The present invention may be used to produce integral monocrystallineextensions on substantially monocrystalline bodies made of any one of avariety of congruently melting materials that solidify in identifiablecrystal lattices. By way of example, the material may be alumina, bariumtitanate, lithium niobate and yttrium aluminum garnet. The invention isalso applicable to other materials that melt congruently (i.e.,compounds that melt to a liquid of the same composition at an invarianttemperature). The following detailed description of the invention andthe apparatus used in practicing the same is directed to growingmonocrystalline extensions of selected geometry onto sapphire tubes.

FIG. 1 shows one form of furnace that may be used to practice theinvention. The furnace consists of a vertically moveable horizontal bed2 which engages a stationary furnace enclosure consisting of twoconcentricspaced quartz tubes 4 and 6. At its bottom end the inner tube4 is positioned in a gasket 5 in the bed. Surrounding tube 4 is a sleeve8 that screws into a collar 10. Between sleeve 8 and collar 10 is anO-ring 12 and a spacer 13. The O-ring is compressed against tube 4 toform a seal. The upper end of sleeve 8 is spaced from tube 4 so as toaccommodate the bottom end of tube 6. The bottom end of tube 6 issecured in place by an O-ring l4 and a spacer l5 compressed between acollar 16 that screws onto sleeve 8. Sleeve 8 is provided with an inletport fitted with a flexible pipe 20. The upper ends of tubes 4 and 6 aresecured in a head 22 so that they remain stationary when the bed islowered. Head 22 has an outlet port with a flexible pipe 24. Althoughnot shown it is to be understood that head 22 includes means similar tosleeve 8, O-rings l2 and 14, and collars l0 and 16 for holding the twotubes in concentric sealed relation. Pipes and 24 are connected to apump (not shown) that continuously circulates cooling water through thespace between the two quartz tubes. The interior of the furnaceenclosure is connected by a pipe 28 to a vacuum pump or to a regulatedsource of inert gas such as argon or helium. The furnace enclosure alsois surrounded by an RF. heating coil 30 that is coupled to acontrollable 500 kc. power supply (not shown) of conventionalconstruction. The heating coil may be moved up or down along the lengthof the furnace enclosure and means (not shown) are provided forsupporting the coil in any selected elevation. At this point it is to benoted that the circulating water not only keeps the inner quartz tube ata safe temperature but also absorbs most of the infrared energy andthereby makes visual observation of crystal growth more comfortable tothe observer.

The head 22 is adapted to provide entry into the furnace enclosure of anelongate pulling rod 32 that is connected to and forms part of aconventional crystal pulling mechanism represented schematically at 34.It is to be noted that the type of crystalpulling mechanism is notcritical to the invention and that the construction thereof may bevaried substantially. Preferably, however, we prefer to employ a crystalpulling mechanism that is hydraulically controlled since it offers theadvantage of being vibration-free and providing a uniform pulling speed.Regardless of its exact construction which is not required to bedescribed in detail, it is to be understood that the pulling mechanism34 is adapted to move pulling rod 32 axially at a controlled rate.Pulling rod 32 is disposed coaxially with the quartz tubes 4 and 6 andits lower end has an extension in the form of metal holder 36 that isadapted to releasably hold a monocrystalline tube 38 on which anintegral monocrystalline extension is to be grown as hereafterdescribed.

Located within the furnace enclosure is a cylindrical heat susceptor 40made of carbon. The top end of susceptor 40 is open but its bottom endis closed off by an end wall. The susceptor is supported on a tungstenrod 42 that is mounted in bed 2. Supported within susceptor 40 on ashort tungsten rod 44 is a crucible 46 adapted to contain a melt 48 ofthe material to be grown onto the tube 38. The crucible is made of amaterial that will withstand the operating temperatures and will notreact with or dissolve in the melt. With an alumina melt, the crucibleis made of molybdenum, but it also may be made of tungsten, iridium orsome other material with similar properties with respect to moltenalumina. Where a molybdenum crucible is used, it must be spaced from thesusceptor since there is a eutectic reaction between carbon andmolybdenum at about 2,200C. The inside of the crucible is of suitablesize and shape, preferably with a constant diameter. To help obtain thehigh operating temperatures necessary for the process, a cylindricalradiation shield 50 made of carbon cloth may be wrapped around thecarbon susceptor. The carbon cloth greatly reduces the heat loss fromthe carbon susceptor.

Referring now to FIGS. 1 and 2, mounted in crucible 46 is a die assembly56 comprising a cylindrical rod 58 that is affixed (e.g. by welding orpress fit) to a supporting disc 60 that rests on a shoulder 62 formed inthe side wall of the crucible at the upper end thereof. The rod has aplurality of axial bores 64 and one or more radial openings 66 near itsbottom end to permit inflow of melt to the several bores from thecrucible. Bores 64 are sized to function as capillaries for moltenalumina. The upper end of rod 58 terminates in a flat horizontal surface68 which intersects the rods outer surface at a right angle It is to benoted that rod 58 projects above disc 60 so as to be visible to theoperator. The length of the rod 58 and diameter of the capillaries 64are such that molten alumina can rise in and fully fill the capillariesby action of capillary rise so long as the level of the melt in thecrucible is high enough to fill the openings 66. The height to which acolumn of melt can rise is determined by the equation h=2Tcos6/drg,where h is the distance in cm. that the column will rise; Tis thesurface tension of the melt in dynes/cm.; 6 is the contact angle, d isthe density of the liquid, r is the internal radius of the capillary incm.; and g is the gravitational constant in cm/sec By way of example ina capillary of 0.75 mm diameter in a molybdenum member, a column ofmolten alumina may be expected to rise more than 11 cm. by capillaryaction.

FIGS. 2-5 illustrate how an end wall of a ceramic material may be grownonto a ceramic tube 38. Initially the tube is inserted vertically and isdisposed in axial alignment with the die assembly. Then with thecapillaries filled with melt by action of capillary rise and the powerinput to coil 30 adjusted so that the upper surface of 68 of the dieassembly is preferably at least about 1040C higher than the meltingpoint of the tube 38, the tube is lowered into contact with the surface68 and held there long enough for a portion of the end of the tube tomelt and form a liquid film 70 that extends laterally far enough toconnect with the melt in the capillaries. It is to be noted that thecapillaries are shown empty in FIGS. 2-5 (and also 6-11) in order torender the capillaries more distinct to the reader and that in fact thecapillaries are filled with melt. Further it is to be understood withreference to FIG. 2 that be fore the end of tube 38 is melted to formfilm 70, the melt in each capillary has a concave meniscus with the edgeof the meniscus being substantially flush with surface 68. Thetemperature gradient along the length of the tube and the temperature ofsurface 68 are factors influencing how much of the tube melts and thethickness of film 70. In this connection it is to be noted that v thetube functions as a heat sink and the temperature of the tube atsuccessively higher points thereon is affected by the height of coil 30and susceptor 40 and also the power input to the coil. In practice theseparameters are adjusted so that the initial film 70 has a thickness inthe order of 0.1 mm.

Once the film 70 has connected with melt in the capillaries, the pullingmechanism 34 is actuated to pull tube 38 upwardly away from surface 58.The pulling speed is set so that the film adhering to the tube becauseof surface tension will crystallize due to a drop in temperature at thesolid tube-liquid film interface which occurs as a result of thepulling. The pulling speed also must be such that surface tension willcause the film to spread inwardly toward the center of surface 68 (seeFIG. 3). As the film spreads inwardly, crystal growth will occur at allpoints along the horizontal expanse of the film with the result that atubular monocrystalline extension is formed on the tube which has aconstant outside diameter but a progressively decreasing insidediameter. The film consumed by the crystal growth is replaced byadditional melt which is supplied by the capillaries 64. Initially thecrystal growth on the tube appears to form a tapered inside flange 72(FIG. 3). As the growth proceeds, the film continues to spread until itfully covers surface 58. Concurrently the flange 72 continues to growinwardly until it completely closes off the tube and forms an end wall72A (FIG. 4). Growth is continued until the end wall 72A has grown tothe desired thickness, where upon the pulling speed is quickly increasedenough to cause the tube to pull free of the film (FIG. 5).Alternatively, growth may be continued so as to causethe wall 72A to beextended into a solid rod having the same outside diameter as the tube.It is to be noted that the pulling speed and the temperature of the filmmay be varied during crystal growth. However, the pulling speed shouldnot be so great nor the temperature so high as to cause the tube to pullfree of the melt film. In growing alpha-alumina, it is preferred to havean initial pulling speed of about 0.1 in/min and to increase the speedto about 0.2 in/min after the film has expanded enough to fully coverthe end surface 68 of the die assembly as above described. The pullingspeed of the tube and the temperature of the film control the filmthickness which controls the rate of film spreading. Increasing thetemperature of surface 68 (and hence the temperature of the film) andincreasing the pulling speed each have the effect of increasing the filmthickness.

FIGS. 6-9 illustrate growth of an internal flange onto the end of aceramic tube. In this case the rod 58 is replaced with a round sleeve58A in which is coaxially disposed a round rod 74. Rod 74 is sized so asto form an annular capillary 64A which functions the same as capillaries64. Sleeve 58A and rod 74 are secured together by a pin 75 and have flatend surfaces 68A and 688 that together function like surface 68 of rod58. Surface 688 has a cylindrical coaxial cavity 76 with a diametercorresponding to the desired internal diameter of the flange to be grownand must be large enough in diameter so that surface tension will notcause the film to close over it. Here again the preformed tube 38 hasthe same outer diameter as surface 68; however, its inside diameter isgreater than the diameter of cavity 76. The procedure followed isessentially the same as described above in connection with FIGS. 2-5.Initially the film 78 that is formed by melting the tube hassubstantially the same inside and outside diameters as the tube. Thisfilm gradually expands inwardly but stops when it reaches cavity 76 (seeFIGS. 7 and 8). As the crystal growth occurs axially, it also expandsinwardly in the same manner as the film so as to form a tapered flange80 (FIG. 7). However, once the film has stabilized at'the edge of cavity76, the crystal growth stops expanding inwardly of the tube andcontinues vertically to the full horizontal expanse of the film, withthe result as shown in FIGS. 8 and 9 that the flange 80 acquires acylindrical inner surface 82. Growth may be discontinued after theflange has developed to a desired thickness as indicated in FIG. 9, orit may be continued so that the grown crystal forms a tube having thesame o.d. but a smaller i.d. than the tube 38. The process of FIGS. 6l0may be carried out with the die assembly of FIG. 2, provided a cavitylike cavity 76 is formed in the upper end of rod 58.

FIGS. 10 and 11 relate to a modification of the process of FIGS. 2-5. Inthis case the upper surface 68 of the rod 58 is concave. The concaveshape may be confined to a circular area bounded by the capillaries 64as shown or may extend out to the edge of surface 68. In either event,when a film 84 is formed and caused to extend fully across surface 68 asabove described, it will fill the concave depression in that surface butwill be relatively flat on top. In other words, the film will tend to bethicker above the low point of the concave depression. Because of theconcave nature of surface 68, the film initially formed by melting thetube will quickly flow to the center of the surface, with the resultthat as the tube is pulled the initial crystal growth will not belimited to the annular region of the film directly in line with the endof the tube but will extend inwardly and will expand quickly to the fullexpanse of the film. Consequently, the inner surface 86 of the end wall88 that is grown onto the end of the tube will be shaped less like acone (see FIG. 4) and more like a shallow dish. When the tube is pulledfree of the film, the outer surface of the end wall 88 will tend to havea contour in vertical section that is somewhat similar to that ofsurface 68. However, the peripheral configuration of the end wall willcorrespond almost exactly to that of surface 68.

In the foregoing modes of practicing the invention, the upper surface ofthe die assembly has substantially the same outer diameter as the tube38, with the result that the monocrystalline flange or end wall and thetube onto which it is grown have substantially the same outsidediameter. However, it also is possible to grow a flange or end wallwhich has a smaller or larger outside diameter. For example, an end wallwith a smaller outside diameter may be grown by using a die assembly ashereabove described having an end surface 68 (as shown in FIG. 2) with asmaller o.d. than the tube 38 (but not smaller than the tubes i.d.);correspondingly, a larger diameter may be grown by using a die assemblywith an end surface 68 (as shown in FIG. 2) that has a larger o.d. thanthe tube. Growing an extension having a larger o.d. and a smaller i.d.than the tube, i.e., an extension that forms both an outside and insideflange on the tube, can be achieved witha die assembly like that shownin FIG. 6 in which the o.d. of surface 68A and the diameter of cavity 76are larger and smaller than the o.d. and i.d. respectively of the tube.

The following example illustrates a preferred mode of practicing theinvention. A molybdenum crucible having an internal diameter of about1%. inch, a wall thickness of about 3/16 inch, and an internal depth ofabout 9/16 inch is positioned in the furnace in the manner shown inFIG. 1. Disposed in the crucible is a die assembly constructed generallyshown in FIG. 2. The rod 58 has four capillaries 64 spaced uniformlyabout its axis. The dimensions of rod 58 are as follows: a rod diameterof about inch and a rod length such that its upper end projects aboutl/l6 inch above the crucible. The four capillaries each have a diameterof about 0.03 inch. The crucible is filled with substantially purepolycrystalline alpha-alumina and a monocrystalline alphaalumina tube 38grown previously by the EFG technique is mounted in holder 38. Tube 38is cylindrical and was grown so that the c-axis of its crystal latticeextends parallel to its geometric axis. Additionally tube 38 has anoutside diameter identical to the diameter of rod 58 and a wallthickness of about 0.03 inch. Tube 38 is mounted in holder 36 so that itis aligned axially with rod 58. Access to seed holder 36 and thesusceptor 40 is achieved by lowering bed 2 away from the furnaceenclosure and lowering the seed holder below the bottom end of furnacetube 4. With the bed restored to the position of FIG. I, cooling wateris introduced between the two quartz tubes. and the enclosure isevacuated and filled with argon to a pressure of about one atomospherewhich is maintained during the growth period. Then the RF. coil 30 isenergized and operated so that the alumina in the crucible is brought toa molten condition (alumina has a melting point in the vicinity of2050C) and the surface 68 reaches a temperature of about 2,070C. As thesolid alumina is converted to the melt 48, columns of the melt will risein and fill capillaries 64. Each column of melt will rise until itsmeniscus is substantially flush with the top of the rod. After affordingtime for temperature equilibrium to be established, the pullingmechanism is actuated and operated so that the tube 38 is moved intocontact with the upper surface 68 of the die assembly and allowed torest in that position to allow the bottom end of the tube to meet andform film 70. After about 60 seconds, the tube is withdrawn verticallyat the rate of about 0.1-0.2 inch per minute. As the tube is withdrawn,crystal growth will occur on the seed and the film of melt will begin tospread over the surface 68 due to its affinity with the newly grownmaterial on the tube and the films surface tension. The latter forcealso causes additional melt to flow out of the capillaries and add tothe total volume of film.

If growth occurs on the tube but the film does not immediately begin tospread, steps are taken to force the melt to spread as desired. This canbe accomplished by adjusting the temperature of the film or by adjustingthe pulling speed. Preferably, the temperature of the surface 68 is heldconstant and the pulling speed adjusted until spreading of the film isobserved. Since the film functions as a growth pool of melt, as the filmspreads out over the surface 68, the growth also expands horizontally.At the aforesaid pulling speed growth will propagate verticallythroughout the entire horizontal expanse of the film, with the resultthat the growing crystal will also begin to grow radially inward asshown in FIGS. 3 and 4 until after about 3 minutes it will conform incross-sectional area and shape to surface 68. As growth continues, theend wall 72A that is formed will be found to have a circular symmetrywith an o.d. substantially the same as that of the surface 68 of the dieassembly. After about 5 minutes of pulling the tube, the pulling speedis immediately increased to about 1.0 inch/minute, whereby the tube 38pulls free of film 70. Thereafter the furnace is cooled and tube 38retrieved from holder 36. The extension grown on the tube is found tohave a reasonably flat bottom surface and a conically shaped interiorsurface as shown in FIG. 5. The thickness of the end wall 72A measuredat the center is about 1 cm. The grown crystal is found to beessentially monocrystalline and a crystallographic extension of thecrystal lattice of tube 38.

It is to be noted that after the film has fully covered surface 68, ifthe operating temperature (as determined by the average temperature offilm is held constant close to but slightly above the melting point ofthe material to be grown, the pulling speed may be varied within limits(depending upon the operating temperature) without any substantialchange in the crosssection of the grown crystal. Similarly if thepulling speed is held constant, the operating temperature may be variedsubstantially (e.g., a change of as much as l530 with respect to themelting point of alumina) without any substantial change in thecross-section of the grown crystal.

The fact that the grown crystalline extension has substantially the sameshape and size as the surface 68 confirms that the film 70 comprises agrowth zone which is substantially isothermal in a direction parallel tosurface 68. It is to be noted that the film has a depth in the order ofabout 0.1 mm. under usual growth conditions and has a verticaltemperature gradient. Surface 68 functions substantially as anisothermal heater. Where the tube has a relatively large o.d. and wallthickness, it may be necessary to increase the rate of heating slightlyso that the temperature of the upper end surface 68 of the die assemblybefore it is contacted by the tube is greater than that normallyrequired to be maintained for satisfactory growth. This highertemperature offsets the heat sink effect of the tube which may cause thegrowth pool, i.e., the film of melt, to have a lower average temperaturethan expected. Unless this heat sink effect is offset by an increase inthe rate of heating, the tube may not melt, or the film may not spreadrapidly over the surface 68, unless the pulling speed is adjusted tocompensate for the heat sink effect.

It is believed obvious that the process of this invention may be used togrow internal or external end flanges on both ends of a tube. Thus usingthe apparatus of FIG. 6 an internal flange may be grown on both ends ofthe tube 38. This is accomplished by (a) growing an internal flange onone end according to the procedure described above and illustrated inFIGS. 6-9, (b) reversing the tube 38 in the holder 36, and (0) growing alike internal flange on the opposite end of the tube according to thesame procedure used to grow the first flange. External end flanges orextensions forming both internal and external flanges or end walls mayalso be grown on both ends of the same tube. Tubes havingmonocrystalline integral end flanges at both ends have utility as lampenvelopes as above described.

It is to be noted also that the invention may be used in growingextensions of other cross-sectional shapes, e.g., rectangular, square,etc., on tubes of the same or different cross-sections. Thus by using adie assembly with a square film-supporting surface. it is possible togrown an extension or termination of square crosssection onto a round orsquare tube.

An important advantage of the invention is that it is applicable tocrystalline materials other than alumina. It is not limited toconqruently melting materials and encompasses growth of materials thatsolidify in cubic, rhombohedral, hexagonal and tetragonal crystalstructures, including barium titanate, yttrium aluminum garnet, andlithium niobate mentioned above. With respect to such other materials,the process is essentially the same as that described above foralpha-alumina, except that it requires different operating temperaturesbecause of different melting points. Additionally, certain minor changesmay be required in the apparatus, e.g., different crucible materials inorder to avoid reaction between the melt and the crucible.

Laue X-ray back reflection photographs of alphaalumina crystal growthproduced according to the foregoing invention reveals that the crystalgrowth usually comprises one or two, and in some cases three or fourcrystals, growing together longitudinally separated by a low angle(usually within 4 of the c-direction) grain boundary. Therefore, forconvenience and in the interest of avoiding any suggestion that thecrystal growth is polycrystalline in character, we prefer to describe itas substantially monocrystalline, it being understood this term isintended to embrace a crystalline body that is comprised of a singlecrystal or two or more crystals, e.g., a bicrystal or tricrystal,growing together longitudinally but separated by a relatively smallangle (i.e., less than about 4) grain boundary. The same term is used todenote the crystalographic nature of the seed tube.

It also has been found that best results are achieved if the c-axis ofthe crystal lattice of the seed tube extends parallel to the tubeslongitudinal axis, so that the extension forming a flange or end wallalso grows vertically along the c-axis. Growth in the c-direction ischaracterized by smooth surfaces and superior strength.

With respect to the die assembly, it is to be understood that in thefollowing claims the term end surface is intended to cover the effectivefilm-supporting surface of the die, whether made as a single piece (seesurface 68 of FIGS. 2 and 10) or as two pieces (see surfaces 68A and 68Bof FIG. 6), and the term capillary is intended to denote a passagewaythat can take a variety of forms, such as the discrete bores 64 or theannular space 64A. The term effective film-supporting surface denotesthe end surface of the die, e.g. surface 68 (or surfaces 68A, 688) as itwould appear if the capillary opening or openings, e.g. capillary 64 (or64A) were omitted, since when a film fully covers the end surface itextends over the capillary openings as shown in FIGS. 4 and 8.

What is claimed is:

1. Method of providing an integral laterallyextending monocrystallineextension on a monocrystalline tube where both said extension and tubeare formed of a congruently melting material comprising providing a dieassembly having an end surface that is larger in at least one directionthan a corresponding dimension of said tube and a capillary that extendsdown from said end surface, filling said capillary with a melt of saidmaterial, contacting said end surface with an end of said tube whilemaintaining said end surface at a temperature at which the end of saidtube will melt and form a film on said surface, melting enough of saidtube end to form a film on said surface that connects with the melt insaid capillary, pulling said tube up away from said surface at a rate atwhich said film will spread over said entire end surface and controllingthe temperature of said film so that crystal growth will occur on saidtube at its interface with said film, simultaneously supplyingadditional melt to said film via said capillary to replenish the meltconsumed by said crystal growth, and terminating crystal growth after adesired amount of crystal growth conforming in cross section tosubstantially the full area of said end surface has occurred on saidtube.

2. Method according to claim 1 wherein said end surface is annular andthe said one end of said tube has substantially the same internaldiameter and a different external diameter than said end surface.

3. Method according to claim 1 wherein said end surface is annular andthe same one end of said tube has substantially the same externaldiameter and a different internal diameter than said end surface.

4. Method according to claim 1 wherein said end surface is annular andthe said one end of said tube has a smaller external diameter and alarger internal diameter than said end surface.

5. Method according to claim 1 wherein said end surface is concave.

6. Method according to claim 1 wherein said tube is pulled at a rate inthe order of 0.l0.2 inch per minute.

7. Method according to claim 1 wherein said end surface has a singleperimeter corresponding in configuration to the outer perimeter of theend of said tube, whereby said crystal growth forms a continuous endwall on said tube.

8. Method according to claim 1 wherein said tube is pulled at oneselected rate until said film has spread across the full expanse of saidend surface, and thereafter is pulled at a faster rate.

9. Method according to claim 1 wherein said end surface has a singleperimeter and is concave and said film spreads out to all points on saidend surface within said perimeter as said tube is being pulled andcrystal growth occurs on said tube.

10. Method of providing an integral lateral extension on amonocrystalline tube where both said tube and extension are formed of acongruently melting material comprising:

providing a die having a generally horizontal end surface and at leastone capillary that extends down from said end surface; filling saidcapillary with a melt of said material and maintaining said end surfaceat a temperature above the melting point of said material;

providing a monocrystalline tube of said material;

positioning said tube above said die and bringing one end of said tubeinto contact with said end surface long enough for a portion of saidtube to melt and form a film on said end surface that connects with themelt in said capillary, thereafter pulling said tube away from said endsurface and simultaneously spreading said film over the full expanse ofsaid end surface while controlling the temperature at the interface ofsaid tube and film so that crystal growth will occur on said tube to thefull area of said surface; and

feeding additional melt to said capillary and via said capillary to saidfilm as said tube is being pulled so to make-up for the materialconsumed by said crystal growth.

1. METHOD OF PROVIDING AN INTEGRAL LATERALLY-EXTENDING MONOCRYSTALLINEEXTENSION ON A MONOCRYSTALLINE TUBE WHERE BOTH SAID EXTENSION AND TUBEARE FORMED OF A CONGRUENTLY MELTING MATERIAL COMPRISING PROVIDING A DIEASSEMBLY HAVING AN END SURFACE THAT IS LARGER IN AT LEAST ONE DIRECTIONTHAN A CORRESPONDING DIMENSION OF SAID TUBE AND A CAPILLARY THAT EXTENDSDOWN FROM SAID END SURFACE, FILLING SAID CAPILLARY WITH A MELT OF SAIDMATERIAL CONTACTING SAID END SURFACE WITH AN END OF SAID TUBE WHILEMAINTAINING SAID END SURFACE AT A TEMPERATURE AT WHICH THE END OF SAIDTUBE WILL MELT AND FORM A FILM ON SAID SURFACE, MELTING ENOUGH OF SAIDTUBE END TO FORM A FILM ON SAID SURFACE THAT CONNECTS WITH THE MELT INSAID CAPILLARY, PULLING SAID TUBE UP AWAY FROM SAID SURFACE AT A RATE ATWHICH SAID FILM WILL SPREAD OVER SAID ENTIRE END SURFACE AND CONTROLLINGTHE TEMPERATURE OF SAID FILM SO THAT CRYSTAL GROWTH WILL OCCUR ON SAIDTUBE AT ITS INTERFACE WITH SAID FILM SIMULTANEOUSLY SUPPLYING ADDITIONALMELT TO SAID FILM VIA SAID CAPILLARY TO REPLENISH THE MELT CONSUMED BYSAID CRYSTAL GROWTH AND TERMINATING CRYSTAL GROWTH AFTER A DESIREDAMOUNT OF CRYSTAL GROWTH CONFORMING IN CROSS SECTION TO SUBSTANTIALLYTHE FULL AREA OF SAID END SURFACE HAS OCCURED ON SAID TUBE.
 2. Methodaccording to claim 1 wherein said end surface is annular and the saidone end of said tube has substantially the same internal diameter and adifferent external diameter than said end surface.
 3. Method accordingto claim 1 wherein said end surface is annular and the said one end ofsaid tube has substantially the same external diameter and a differentinternal diameter than said end surface.
 4. Method according to claim 1wherein said end surface is annular and the said one end of said tubehas a smaller external diameter and a larger internal diameter than saidend surface.
 5. Method according to claim 1 wherein said end surface isconcave.
 6. Method according to claim 1 wherein said tube is pulled at arate in the order of 0.1-0.2 inch per minute.
 7. Method according toclaim 1 wherein said end surface has a single perimeter corresponding inconfiguration to the outer perimeter of the end of said tube, wherebysaid crystal growth forms a continuous end wall on said tube.
 8. Methodaccording to claim 1 wherein said tube is pulled at one selected rateuntil said film has spread across the full expanse of said end surface,and thereafter is pulled at a faster rate.
 9. Method according to claim1 wherein said end surface has a single perimeter and is concave andsaid film spreads out to all points on said end surface within saidperimeter as said tube is being pulled and crystal growth occurs on saidtube.
 10. Method of providing an integral lateral extension on amonocrystalline tube where both said tube and extension are formed of acongruently melting material comprising: providing a die having agenerally horizontal end surface and at least one capillary that extendsdown from said end surface; filling said capillary with a melt of saidmaterial and maintaining said end surface at a temperature above themelting point of said material; providing a monocrystalline tube of saidmaterial; positioning said tube above said die and bringing one end ofsaid tube into contact with said end surface long enough for a portionof said tube to melt and form a film on said end surface that connectswith the melt in said capillary, thereafter pulling said tube away fromsaid end surface and simultaneously spreading said film over the fullexpanse of said end surface while controlling the temperature at theinterface of said tube and film so that crystal growth will occur onsaid tube to the full area of said surface; and feeding additional meltto said capillary and via said capillary to said film as said tube isbeing pulled so as to make-up for the material consumed by said crystalgrowth.